Electrical power steering systems include an electrical motor and a controller having inverter drive circuitry. The combination of the electrical motor and the controller is sometimes referred to as an electric actuator. The electric actuator rotates in both directions of operation. That is, the electric actuator may operate in a clockwise as well as a counterclockwise direction. The electric actuator also operates to produce torque in both directions as well. Thus, the electric actuator operates in all four quadrants of operation for a motor, which means that motor torque and motor velocity may each be positive or negative, resulting in four possible combinations of operation.
In the event that the electric actuator is operating in either quadrant 2 or quadrant 4, the electric actuator operates as a generator. That is, in the event the motor torque and the motor velocity have opposing signs (i.e., positive or negative), a supply current may become negative. The negative supply current is also referred to as a regenerative current. For example, in the event the electric actuator is connected to a vehicle battery, the regenerative current may be sent to the vehicle battery. Over time, vehicle battery performance decreases. Therefore, if battery performance is diminished or if a vehicle electrical system issue arises, the regenerative current may not be absorbed by the vehicle battery. Thus, there exists a need to limit the amount of regenerative current that is produced by the electric actuator when operating in either quadrant 2 or quadrant 4. In one approach to reduce the regenerative current to the vehicle battery, passive elements such as, for example, a resistive element may be used to dissipate the regenerative current. However, passive elements may be large and difficult to package.
In addition to reducing the regenerative current, the electric motor typically has torque versus speed requirements that need to be met for quadrant 1 and quadrant 3. Thus, an approach referred to as phase advance may be employed to meet torque versus speed requirements, which results in an increased amount of power generation from the electric motor. Phase advance involves allowing the phase of an applied motor voltage to shift versus a phase of a developed motor back electromotive force (BEMF). In the event that phase advance is not required to meet torque versus speed requirements in quadrant 2 and quadrant 4, a zero phase advance value may be used. This approach results in relatively simple software for the controller. However, a relatively large amount of motor current may be developed (i.e., in one example motor current may reach as high as about 220 Amps), which may create electrical stress on the internal circuitry of the controller, and may also create noise issues as well.
In another approach, phase advance may be used in quadrant 2 and quadrant 4 to meet torque versus speed performance requirements. In this approach, a phase advancement angle is calculated which causes a d-axis or field current of the electrical motor to be about zero in quadrant 2 or quadrant 4. This approach results in a lower amount of motor current that is developed, which in turn reduces the amount of electrical stress on the internal circuitry of the controller. However, this approach also results in relatively large amounts of regenerative current that may be produced.